1. Field of the Invention
The present invention relates to a heat exchanger with a heat pipe.
2. Description of the Related Art
JP-A-4-45393 describes a heat exchanger using a loop type heat pipe, which is used for a hot water supply system. In the heat exchanger using the loop type heat pipe, a working fluid is circulated in a closed loop circuit to be evaporated and to be condensed. The working fluid is evaporated in an evaporation portion by absorbing heat from a thermal storage material, and is condensed in a condensation portion by radiating heat to water. In this heat exchanger, lower end portions of plural heat pipes communicate with each other by a lower communication portion (lower header), such that the working fluid condensed in the condensation portion flows into the plural heat pipes via the lower communication portion.
Furthermore, in a heat-pipe type heat exchanger, inner fins are located within heat pipes in order to increase a heat transmission area between the heat pipes and a working fluid and to improve pressure-resisting strength. Alternatively, in a case where inner fins are not provided in a heat-pipe type heat exchanger in which plural heat pipes are stacked and brazed, ribs are formed to improve brazing performance and pressure-resisting strength. Specifically, the heat pipe is provided with an inner fluid passage by oppositely arranging a first flat plate and a second flat plate, and the ribs protruding to the inner fluid passage are formed in the first flat plate and the second flat plate to contact at its tip ends.
The heat-pipe type heat exchanger is generally used for recovering heat of exhaust gas of an internal combustion engine of a vehicle so as to improve an engine-heating performance by using the exhaust heat. The temperature of exhaust gas may be increased to 900° C. in maximum. In contrast, water is normally used as the working fluid, and water is solidified (freeze) so that the volume expansion of about 9% is caused as compared with that in a liquid state. However, when water is used as the working fluid, the following problems may be caused.
For example, in a case where water is used as the working fluid in a heat exchanger having heat pipes provided with inner fins, when the working fluid is solidified at a low temperature condition such as the freeze point, the solidification of the working fluid starts firstly at portions contacting the heat pipes, the lower communication portion and the inner fins. FIG. 11 shows a heat-pipe type heat exchanger 300A including a heat pipe 303a, inner fins 308 located in the heat pipe 303a, and a lower communication portion 305b. As shown in FIG. 11, when liquid working fluid B is closed by solid working fluid A in accordance with progress of the solidification of the working fluid, inner pressure of the heat exchanger 300A is increased by solidification of the closed liquid working fluid B thereby pressure applied to the heat pipe 303a and the lower communication portion 305b is increased. When the top surface of the working fluid contacts the inner fins 308, the liquid working fluid B closed in the solid working fluid A becomes larger because the top portion of the working fluid contacting the inner fins 308 is solidified at an early time in the solidification process. In this case, the inner pressure applied to the heat pipe 303a and the lower communication portion 305b will be further increased if the liquid working fluid B closed by the solid working fluid A is solidified.